The present invention relates to the field of discharge lighting and, in particular, to efficiently supplying electrical power for driving a discharge lamp, such as used to backlight a color liquid crystal display (LCD) panel, by controlling an alternating current signal that is generated from a range of direct current signals.
A discharge lamp used to backlight an LCD panel such as a cold cathode fluorescent lamp (CCFL) has terminal voltage characteristics that vary depending upon the immediate history and the frequency of a stimulus (AC signal) applied to the lamp. Until the CCFL is xe2x80x9cstruckxe2x80x9d or ignited, the lamp will not conduct a current with an applied terminal voltage that is less than the strike voltage, e.g., the terminal voltage must be equal to or greater than 1500 Volts. Once an electrical arc is struck inside the CCFL, the terminal voltage may fall to a run voltage that is approximately ⅓ the value of the strike voltage over a relatively wide range of input currents. For example, the run voltage could be 500 Volts over a range of 500 microAmps to 6 milliAmps for a CCFL that has a strike voltage of 1,500 Volt. When the CCFL is driven by an AC signal at a relatively low frequency, the CCFL""s electrical arc tends to extinguish and ignite on every cycle, which causes the lamp to exhibit a negative resistance terminal characteristic. However, when the CCFL is driven by another AC signal at a relatively high frequency, the CCFL (once struck) will not extinguish on each cycle and will exhibit a positive resistance terminal characteristic. Since the CCFL efficiency improves at the relatively higher frequencies, the CCFL is usually driven by AC signals having frequencies that range from 50 KiloHertz to 100 KiloHertz.
Also, the mean time between failure for a CCFL is dependent upon several aspects of the operating environment. For example, driving the CCFL at a power level that is higher than the rated power level tends to shorten the useful lifetime of the lamp. Also, driving the CCFL with an AC signal that has a high crest factor can cause premature failure of the lamp. The crest factor is the ratio of the peak current to the average current that flows through the CCFL. Additionally, it is known that driving a CCFL with a relatively high frequency square-shaped AC signal will produce the maximum useful lifetime for the lamp. However, since the square shape of an AC signal may cause significant interference with another circuit disposed in the immediate vicinity of the circuitry driving the CCFL, the lamp is typically driven with an AC signal that has a less than optimal shape such as a sine-shaped AC signal.
Most small CCFLs are used in battery powered systems, e.g., notebook computers and personal digital assistants. The system battery supplies a direct current (DC) voltage ranging from 7 to 20 Volts with a nominal value of about 12V to an input of a DC to AC inverter. A common technique for converting a relatively low DC input voltage to a higher AC output voltage is to chop up the DC input signal with power switches, filter out the harmonic signals produced by the chopping, and output a relatively clean sine-shaped AC signal. The voltage of the AC signal is stepped up with a transformer to a relatively high voltage, e.g., from 12 to 1500 Volts. The power switches may be bipolar junction transistors (BJT) or Field Effect Transistors (FET or MOSFET). Also, the transistors may be discrete or integrated into the same package as the control circuitry for the DC to AC converter.
Since resistive components tend to dissipate power and reduce the overall efficiency of a circuit, a typical harmonic filter for a DC to AC converter employs inductive and capacitive components that are selected to minimize power loss, i.e., each of the selected components should have a high Q value. The Q value identifies the xe2x80x9cquality factorxe2x80x9d of an inductor or a capacitor by indicating the ratio of energy stored to energy lost in the component for a complete cycle of an AC signal at a rated operational frequency. The Q value of a component will vary with the frequency and amplitude of a signal, so a filter must be designed for minimum (or acceptable) loss at the operating frequency and required power level. Also, some DC to AC converter filters incorporate the inductance of the step-up transformer, either in the magnetizing inductance of the primary or in the leakage inductance of the secondary.
A second-order resonant filter formed with inductive and capacitive components is also referred to as a xe2x80x9ctankxe2x80x9d circuit because the tank stores energy at a particular frequency. The unloaded Q value of the tank may be determined by measuring the parasitic losses of the tank components, i.e., the total energy stored by the tank for each cycle of the AC signal is divided by the total energy lost in the tank components each cycle. A high efficiency tank circuit will have a high unloaded Q value, i.e., the tank will employ relatively low loss capacitors and inductors.
The loaded Q value of a tank circuit may be measured when power is transferred through the tank from an energy source to a load, i.e., the ratio of the total energy stored by the tank in each cycle of the AC signal divided by the total energy lost in the tank plus the energy transferred to the load in each cycle. The efficacy of the tank circuit as a filter depends on its loaded Q value, i.e., the higher the loaded Q value, the purer the shape of the sine wave output. Also, the efficiency of the tank circuit as a power transmitter depends on the ratio of the unloaded Q to the loaded Q. A high efficiency tank circuit will have an unloaded Q set as high as practical with a loaded Q set as low as possible. Additionally, the loaded Q of the tank circuit may be set even smaller to increase the efficiency of the filter, if the signal inputted to the tank has most of its energy in a fundamental frequency and only a small amount of energy is present in the lower harmonic frequencies.
The energy of a periodic waveform may be assigned to discrete frequencies, i.e., the fundamental repetition frequency and integer multiples of the fundamental repetition frequency. The fundamental repetition frequency is referred to as the fundamental and the integer multiples are termed harmonics. Generally, waveforms with sharp edges have fast rise and fall times and they have more energy in high order harmonics than waveforms with smooth edges and relatively slow transitions. However, generating waveforms with smooth, slow transitions usually causes fairly high power dissipation in the chopping switches, so the actual waveform is usually a compromise between efficient (sharp), fast edges and quiet (smooth), slow edges. Waveforms that are symmetric, i.e., the up-going waveform shape is the mirror image of the down-going shape but shifted in time, tends to suppress or cancel the even harmonics, which are the fundamental frequency times the integer values of 2, 4, 6, 8, 10, etc. The suppression or cancellation of the even harmonics is important because the harmonic frequency closest to the fundamental frequency is the second harmonic, which is the most difficult harmonic frequency to filter out of the waveform.
The largest component in a small DC to AC inverter circuit for a CCFL is the step-up transformer. Typically, this transformer includes a primary and a secondary winding coiled around a plastic bobbin mounted to a ferrite core. This type of transformer has two characteristic inductances associated with each winding, i.e., a magnetizing inductance and a leakage inductance. The value of the magnetizing inductance for each winding is measured when the other winding is configured as an open circuit, i.e., a no load state. Also, the value of the leakage inductance for each winding is measured when the other winding is configured as a short circuit.
The magnetizing inductance of a winding is a measure of how well the particular winding is coupled to the core of the transformer, i.e., a large magnetizing inductance is an indication that the magnetic flux of the winding is mostly contained within the core. A gap in the core will lower the magnetizing inductance because all of the magnetic flux is forced to leave the core at the gap. Thus, a relatively small transformer may be used to deliver a given power level, if the core is not gapped.
The leakage inductance is a measure of how poorly a winding is coupled to the other winding, i.e., a large leakage inductance indicates when the other winding is shorted. Since a high voltage, e.g., a strike voltage of 1500 volts, may be impressed on the secondary winding of the transformer for a CCFL converter, relatively thick insulators are typically used between the primary and the secondary windings. However, thick insulators tend to cause the leakage inductances of the primary and secondary windings to be relatively large.
The intensity of light emitted by a CCFL may be dimmed by driving the lamp with a lower power level (current). Dimming the light emitted by the CCFL enables the user to accommodate a wide range of ambient light conditions. Because the CCFL impedance will increase as the power level driving the lamp is reduced, i.e., an approximately constant voltage with a decreasing current, currents in the stray capacitances between neighboring conductors (e.g., ground shields, wiring) and the lamp tend to become significant. For example, if the control circuitry requires that one terminal of the CCFL is tied to signal ground for measuring current through the lamp, the current in the grounded terminal of the lamp will be significantly less than the current flowing into the other terminal of the lamp. In this case, a thermometer effect on the CCFL will be produced, whereby the grounded end of the lamp has almost no current flowing in it and the arc essentially extinguishes while the other end of the lamp is still arcing and emitting light. The thermometer effect may be greatly reduced by the technique of driving the CCFL, so that the signal at one end of the lamp is equal to and exactly out of phase with the signal at the other end. This technique is typically termed a balanced drive and it may be approximated by driving the CCFL with a floating secondary winding, i.e., neither end of the secondary winding is tied to ground.
The invention is a method and apparatus for efficiently converting a direct current (DC) signal into an alternating current (AC) signal for driving a load such as a discharge lamp. A network of a plurality of switches converts a DC signal coupled to the network into an AC signal. A tank circuit is coupled between the network of the plurality of switches and the discharge lamp. The tank circuit filters and smoothes the AC signal that is transmitted from the network of the plurality of switches to the discharge lamp. A controller employs a resonant frequency of the tank circuit to control the oscillation of the network of the plurality of switches between the open and closed positions. Since the network of the plurality of switches oscillates at the resonant frequency of the tank circuit, the AC signal drives the discharge lamp with the optimal amount of electrical power over a range of AC signal voltages. Additionally, the network of the plurality of switches and the controller may be disposed in a monolithic integrated circuit.
The tank circuit includes a step-up transformer with a primary winding that receives the AC signal from the network of the plurality of switches and a secondary winding that is coupled to the discharge lamp. The ratio of the primary winding and the secondary winding causes an AC signal with a relatively higher voltage to be induced across the secondary winding than the AC signal transmitted to the primary winding. The tank circuit includes a filter for the AC signal. The filter may be disposed between the network of the plurality of switches and the primary winding of the step-up transformer. Alternatively, the filter may be positioned between a secondary winding of the step-up transformer and the load.
The filter may be a second order filter that includes an inductor and a capacitor. The filter provides for suppressing a harmonic signal associated with the AC signal and smoothing the AC signal""s waveform.
The plurality of switches may be MOSFETs that are arranged in an H-bridge network.
A zero crossing detector determines the resonant frequency of the tank circuit by indicating to the controller the zero crossing point of the current in the tank circuit. This indication is used by the controller to follow the frequency response of the tank circuit by providing an indication of the zero crossing point of the tank circuit""s resonant frequency in real time. This indication is used by the controller to follow the frequency response of the tank circuit when the amount of loading presented by the discharge lamp has caused the circuit""s resonant frequency to shift away from an initial, i.e., unloaded, resonant frequency.
The load may be a discharge lamp, including a cold cathode fluorescent, metal halide and sodium vapor.
A brightness control may be provided for enabling a user to dim the amount of light emitted by the discharge lamp. Also, a loop compensation capacitor may have an end connected to a voltage reference and another end coupled to an on-time timer, the brightness control and the controller. The voltage impressed across the loop compensation capacitor is used by the timer to set the xe2x80x9conxe2x80x9d time for each power phase of the plurality of switches. Also, the loop compensation capacitor""s voltage is set by a feedback loop that compares a DC voltage modulated by the brightness control with either the amount of current or power delivered to the discharge lamp.
The controller uses the voltage impressed across the loop compensation capacitor to determine the time interval of the on-time timer for each power phase. Also, the controller employs the zero crossing detector to determine when to switch to the next phase of the cycle and begin injecting an amount of energy into the tank circuit. In the event of a conflict between the time interval of the on-time timer and the indication provided by the zero crossing detector, i.e., the detector provides the indication before the time interval is over, the detector wins and controller will cause the H-bridge components to switch to the next phase of the cycle.
The controller implements several logical determinations that, if true, will cause the controller to stop the AC signal from driving the load, including: (1) determining if an undervoltage condition is occurring at the battery supply; (2) determining when a thermal overload condition is occurring; and (3) determining if the load current has exceeded a predetermined maximum value. The controller may also determine if an on mode is selected, and if so enable the AC signal to drive the load.
Additionally, the controller may respond to a burst mode signal from the user, and if so the controller switches the H-bridge MOSFETs xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d at a user-determined burst mode frequency that is substantially lower than the AC signal driving the load, e.g., the AC signal driving the discharge lamp may have a 50 KiloHertz frequency and the burst mode switching (on and off periods) may occur at a 180 Hertz frequency. The loop compensation capacitor is neither charged nor discharged during the burst mode off period so that the on-time period of the timer is xe2x80x9crememberedxe2x80x9d for use in the next on period.
Since the burst mode reduces the total amount of energy that the discharge lamp is receiving, the amount of light emitted by the lamp is dimmed. Also, burst mode switching enables the discharge lamp to be dimmed without having to compensate for stray capacitances between the leads of the discharge lamp. Analog dimming may be used in combination with burst mode switching to provide an even larger range of dimming for the discharge lamp.
A gate driver may be provided for each MOSFET in the H-bridge network. The gate driver amplifies logic signals that control the operation of the associated MOSFET. Also, the gate driver may provide a lockout mode of operation that prevents the associated MOSFET from cross conducting with another MOSFET. The gate drivers are used to limit current in case of an open lamp condition and a terminal short to ground.
A capacitor may be provided with an end connected to a voltage reference and another end coupled to an output terminal of the H-bridge network and the load. The capacitor provides energy for an upper MOSFET""s gate driver when a turn on voltage is applied to a gate of the upper MOSFET. In this case, the voltage at a source of the upper MOSFET is approximately equal to the input supply voltage when the MOSFET is on. Additionally, the gate driver may provide for initially charging the capacitor before the load is driven by the AC signal. Further, the gate driver may recharge the capacitor when the MOSFET associated with gate driver is not conducting.
The oscillation of the plurality of switches based on a resonant frequency of the tank circuit is performed in a predetermined cycle. In a first power phase of the cycle, a portion of the plurality of switches is turned on to supply a portion of the AC signal. In a second power phase of the cycle, another portion of the plurality of switches is turned on to generate an opposite portion of the AC signal. The plurality of switches oscillate between the first and second power phases. Additionally, the cycle may cause the plurality of switches to exit the first power phase and enter a first rest phase, and exit the second power phase and enter a second rest phase. The AC power delivered to the load may be varied by changing the ratio of the time spent in the power phases versus the time spent in the rest phases of the cycle.
The gate driver may determine if the flow of current through the associated MOSFET is equal to or greater than a predetermined value. If true, the associated MOSFET will be turned off for the current power phase, either the first power phase or the second power phase, until the start of the next power phase.
The method provides for substantially the same functionality of the apparatus, albeit in ways that may differ.